How can one measure the speed of light in MPH? so, how can we actually do it?
Its well known that the speed of light is a certain number of miles per second, but if its never been tested, how do we get that number?
 A: Its pretty straight forward

Measuring the Speed of Light from Astronomical Observations
Roemer was the first person to come up with a number for the speed of
  light. He did it while observing the eclipses of of Jupiter's moons,
  specifically Io. He would watch Io disappear behind the giant planet
  and then time how long it took to reappear. He reasoned that this time
  could differ by as much as 1,000 seconds, depending on how close
  Jupiter was to the earth. He came up with a value for the speed of
  light of 214,000 km/s, which is in the same ballpark as the modern
  value of almost 300,000 km/s.
In 1728, English astronomer James Bradley calculated the speed of
  light by observing stellar aberrations, which is their apparent change
  in position due to the earth's motion around the sun. By measuring the
  angle of this change and subtracting the speed of the earth, which he
  could calculate from data known at the time, Bradley came up with a
  much more accurate number. He calculated the speed of light in a
  vacuum to be 301,000 km/s.

A: You can't measure it! Not since 1983. The speed of light is defined to be 299,792,458 metres per second - because in the SI system  the metre is defined to be the distance light travels in 1/299792458 of a second. A mile is nowadays defined to be 1,609.344 metres, and an hour is just 3600 seconds, so that's  670,616,629 mph. 
So you can't measure it. It's defined. These undergraduate experiments with laser beams are actually calibrating the metre rules and/or clocks used in the apparatus.
$c=299,792,458 ms^{-1}$. That's not a measurement. That's the law.
